Many base station antennas used for commercial communications, e.g., cellular service, are omni-directional. One such cellular base station antenna is a co-axial, sleeve dipole collinear vertical antenna array manufactured by The Antenna Specialists Co., a division of Orion Industries, Inc., the assignee of this application. This type of antenna includes a stacked array of elongated radiators, e.g., a "dumbbell" like sections, which constitute a vertical array of collinear sleeve dipole radiators. The array is center fed by a concentric co-axial feed structure.
At the approximate center of the stacked antenna array, the co-axial feed structure is terminated by connection to the adjacent one of the intermediate radiating elements. The location of the feed point affects desired phasing relative to propagation through the stacked dipole radiator array above and below the feed point connection. By changing the location of the tap or connection points to the array, the beam tilt of the major lobe can be controlled. In this way, antennas have been constructed with different amounts of downward or negative beam tilt, typically at angles of between about -3.degree. and about -8.degree..
Good radiation coverage from such antennas results not only from an appropriate gain antenna, but also is a function of directing radiation into areas where coverage is desired. Since, for example, antennas for cellular service are typically used for short distance communications with mobile units located below the antenna site, downwardly directed beams having negative beam angles, are normally utilized. As is known, controlling the phasing of the elements of the stacked array is effective to aim the vertical beam downwardly at an angle relative to the horizontal. The feeding of spaced dipole elements with controlled phase variances electrically tilts the beam downwardly at an angle to the axis of the radiators to effectuate the desired coverage.
Different antenna sites or installation locations may advantageously utilize antennas producing radiation patterns having different downward beam tilt angles. Factors bearing on beam angle selection include position, height, and the environment in which the antenna is operating. Thus, different downward beam tilt angles may be appropriate for an antenna installed in an urban area in a relatively high position and an antenna installed in a less populated area at a different height.
Different antennas with different beam angles have been used where different beam tilt is desired. Each such antenna is designed and constructed to provide a single selected beam tilt angle.
It would be desirable to be able to provide an antenna with a variable beam tilt capability which would have the flexibility of adjustable beam tilt and yet be simple to set up and adjust both prior to or after the antenna is installed.